1. Field of the Invention
The present invention relates to a technique preferably used in the field of photo-electronics (photonics), such as optical communication and optical information processing, including a thermal lens forming optical element, a deflection element having the thermal lens forming optical element, an optical control method using the thermal lens forming optical element, and an optical control apparatus equipped with the thermal lens forming optical element. More particularly, the present invention relates to a technique capable of causing deflection of light (signal light) based on a change in a refractive index of the thermal lens forming optical element, a deflection element having the thermal lens forming optical element, an optical control method using the thermal lens forming optical element, and an optical control apparatus equipped with the thermal lens forming optical element.
Moreover, the present invention relates to a deflection-type optical path switching apparatus and an optical path switching method preferably used in the optical communication field and the optical information processing field.
2. Description of the Invention
The deflection of light can be caused by:    (1) mechanically tilting a mirror;    (2) mechanically rotating a polygonal mirror;    (3) acoustooptic effects; or    (4) electro-optical effects.
The above-listed method (1) using a mechanically tilting mirror requires an expensive control mechanism to realize accurate deflection and cannot be used at higher frequencies. The above-listed method (2) using a rotatable polygon mirror requires higher costs. The above-listed methods (3) using acoustooptic effects and (4) using electro-optical effects both require higher costs and larger scale devices, yet are only capable of creating relatively small deflection angles.
Modulation of light can be realized by generating a temperature-dependent refractive index distribution in a medium (refer to Japanese Patent Application Laid-Open No. 60-14221). The method discussed in this art includes heating a medium using a heat-generation resistor, generating a refractive index distribution in the medium, and causing deflection of light. The deflected light is selectively shielded by a light-shielding plate to realize flickering of a light spot. However, the method discussed in the above-noted prior art is not free from a “divergence of heat” problem because the heat-generation resistor (i.e., heat generation element) relies of heat conduction to heat a medium. More specifically, the “divergence of heat” deteriorates the formation of a finely controlled heat gradient in a wide area. Accordingly, a desired refractive index distribution cannot be obtained. Furthermore, a photolithography technique, which is preferably employed in the manufacturing of semiconductor integrated circuits, cannot be used to finely machine or process a heat-generation resistor. Due to such practical restrictions, the size of an element tends to become larger. If an element size is increased, an optical system will be structurally complicated and enlarged correspondingly. Furthermore, when a heat-generation resistor is used as a heat-generation element, the response is slow because the temperature increase of the medium is dependent on heat conduction. Furthermore, as an inherent problem, the refractive index change at higher frequencies cannot be attained.
Furthermore, a laser can be used to heat a material and change a refractive index of the material for deflection of the laser beam (refer to U.S. Pat. Nos. 4,776,677 and 4,585,301). The methods discussed in the above-noted prior art documents require a high power laser having a large beam size to create desirable deflection of the laser beam. The method discussed in U.S. Pat. No. 4,776,677 can cause deflection of emitted light by heat generated by the emitted light itself. If the method discussed in U.S. Pat. No. 4,776,677 is used for optical deflection, the emitted light is almost absorbed to heat a material and change a refractive index. According to the principle, only a small quantity of light can pass through the material.
The method discussed in U.S. Pat. No. 4,585,301 uses no electrical or mechanical mechanism. An optical switch disclosed in the U.S. Pat. No. 4,585,301 can change a refractive index with an emitted control beam and change an optical path of a signal beam. However, in this case, the laser must have a large power to change the refractive index because no lens is used for beam-condensation of the control beam and the signal beam. Furthermore, the apparatus has a larger size. Moreover, the deflection angle cannot be enlarged because there is no mechanism for expanding a refraction change region according to traveling of a beam as proposed in the present invention.
Furthermore, a deflection element can include a thermal lens forming optical element containing a photoreactive composition and an intensity distribution adjustment mechanism for irradiating the thermal lens forming optical element with a beam having a wedge-shaped light intensity distribution (refer to Japanese Patent Application Laid-Open No. 11-194373). The refractive index distribution can be formed in the thermal lens forming optical element with control light. Deflection of signal light can be realized by using the refractive index distribution. The wavelength of the signal light differs from that of the control light. Although it is excellent to use light to control light, the above-described system requires a highly-advanced adjustment technique for the intensity distribution adjustment mechanism that irradiates the thermal lens forming optical element with a beam having a wedge-shaped light intensity distribution. Even a slight change in the control light intensity will result in a large change in the deflection angle.
The widespread use of the Internet and corporate and home networks has meanwhile led to a rapid increase in network traffic. Thus, an optical path switching apparatus including no intervening electric signal (optical switch), i.e., a light-light direct switch, is desired. A practical apparatus and method for switching an optical path, e.g., an optical fiber, an optical waveguide, or a route of light traveling in a space, can be a space division type according to which an optical path is switched in an optical waveguide or between optical waveguides, a wavelength-division multiplex type according to which wavelength multiplexed light is divided and switched to optical paths corresponding to respective wavelengths, a time-division multiplex type according to which time-division multiplexed light is switched to a corresponding optical path at predetermined timing, or a free-space type according to which an optical path of light propagating in a space is spatially divided/mixed using a mirror or a shutter. Multiplexing or combining the above-described apparatus/methods is also feasible.
The space division type optical switch can be an optical switch utilizing a directional coupler, an optical switch that forms a copy of a light signal using a light branch unit and turns on/off the light with a gate element, or an optical switch that changes a refractive index of a waveguide at a crossing or Y-branch portion to selectively transmit or reflect the light propagating in the waveguide, although these switches are in a research and development stage. To change a refractive index of a waveguide of a Mach-Zehnder interferometer-type optical waveguide switch, an optical switch using thermo-optical effects obtainable from heat generation by an electric heater is almost practically usable. However, not only the response speed is as low as 1 msec but also an electric signal is used for the action of an optical switch.
The free-space type optical switch can be a micro electro mechanical system (MEMS), an exciton absorption reflection switch (EARS switch), a multi-stage beam shifter-type optical switch, a hologram-type optical switch, or a liquid crystal switch. However, these switches cannot be practically used because of a mechanically movable portion and polarization dependency.
On the other hand, transmissivity or refractive index can be changed by irradiating a thermal lens forming optical element with light. The research for developing a full-light type thermal lens forming optical element using such changes to directly modulate the intensity or frequency of light with light, or a related optical control system, has been widely conducted. For the purpose of developing new information processing technique based on a full-light type optical element, the inventors of the present invention have enthusiastically conducted a research for an optical control system using an organic nano-particle optical thermal lens forming element containing organic dye aggregate diffused in a polymer matrix (refer to T. Hiraga, N. Tanaka, K. Hayami, and T. Moriya: “generation, structural evaluation, and photophysics of dye associate and aggregate”, Electronic Science and Technology Report, Vol. 59, No. 2, pp. 29-49 (1994), published by National Institute of Advanced Electronic Science and Technology, Agency of Industrial Science and Technology, Ministry of International Trade and Industry). An element presently developed by the inventors of the present invention can modulate signal light (780 nm and 1550 nm) with control light (660 nm and 980 nm) which are coaxially input to the same focus point, based on the principle that a thermal lens transiently formed by the absorption of the control light can refract the signal light. A high-speed response equivalent to approximately 20 nano-sec has been attained. There is an optical control method including emitting control light to a thermal lens forming optical element containing a photoreactive composition, reversibly changing transmissivity and/or refractive index of signal light differentiated in wavelength band from the control light, and performing intensity modulation and/or light flux density modulation for the signal light passing through the thermal lens forming optical element. For example, there is an optical control method characterized by emitting respectively converged control light and signal light into a thermal lens forming optical element and adjusting optical paths of the control light and the signal light in such a manner that highest photon density regions of the control light and the signal light in the vicinity of their focus points (beam waist) are overlapped with each other in the thermal lens forming optical element (refer to Japanese Patent Application Laid-Open No. 8-286220, Japanese Patent Application Laid-Open No. 8-320535, Japanese Patent Application Laid-Open No. 8-320536, Japanese Patent Application Laid-Open No. 9-329816, Japanese Patent Application Laid-Open No. 10-90733, Japanese Patent Application Laid-Open No. 10-90734, and Japanese Patent Application Laid-Open No. 10-148852). Furthermore, there is an optical control method including emitting control light and signal light having mutually different wavelengths into a thermal lens forming optical element containing a photoreactive composition, wherein the wavelength of the control light is selected from a wavelength band that the photoreactive composition can absorb, reversibly forming a thermal lens based on a distribution of density change caused by a temperature increase in a region where the photoreactive composition can absorb the control light or its peripheral region, and performing intensity modulation and/or light flux density modulation for the signal light passing through the thermal lens (refer to Japanese Patent Application Laid-Open No. 10-148853). A practical thermal lens forming optical element is, for example, a dye/resin film or a dye solution film. When the control light is emitted at a power level in the range from 2 through 25 mW, the response time of signal light is less than 2 μsec (refer to Japanese Patent Application Laid-Open No. 10-148853). The aforementioned methods are excellent in the capability of controlling light with light and increasing the response speed. However, a light flux shape formed when the control light is emitted becomes a doughnut shape and accordingly the coupling efficiency to an optical fiber is small.
The thermal lens effect is a phenomenon caused by a molecular that absorbs energy of light and converts light to heat at a central portion of an irradiated portion. The generated heat is propagated to the surrounding area. A temperature distribution is generated. As a result, the refractive index of a light transmission medium changes with a spherical shape from the light absorption center toward the external portion. The light absorption center has a lower refractive index, and the external portion has a higher refractive index. This distribution can produce light refraction effects similar to those of a concave lens. The thermal lens effect has been long utilized in the field of spectral analysis. A supersensitive spectral analysis available at present can detect a light absorption by a single molecular (refer to K. Fujiwara, K. Fuwa, and T. Kobayashi: laser-induced thermal lens effect and its application to colorimetric method, “Chemistry”, published by Kagakudojin, Vo. 36, No. 6, pp 432-438 (1981), or T. Kitamori and T. Sawada: photothermo-conversion spectroscopy, “Bunseki”, published by the Japan Society for Analytical Chemistry, March 1994, pp 178-187).
There is a method for realizing deflection of an optical path using a refractive index change caused by thermal lens effects or heat. According to this method, the temperature of a medium is increased by a heat-generation resistor so that deflection of light can be realized according to a change of refractive index distribution in the medium (refer to Japanese Patent Application Laid-Open No. 60-14221). However, the method discussed in the above-mentioned prior art is not free from a “divergence of heat” problem because the heat-generation resistor (i.e., heat generation element) relies on heat conduction to heat a medium. More specifically, the “divergence of heat” deteriorates formation of a finely controlled heat gradient in a wide area. Accordingly, a desired refractive index distribution cannot be obtained. Furthermore, a photolithography technique, which is preferably employed in the manufacturing of semiconductor integrated circuits, cannot be used to finely machine or process a heat-generation resistor. Due to such practical restrictions, the size of an element tends to become larger. If an element size is increased, an optical system will be structurally complicated and enlarged correspondingly. Furthermore, when a heat-generation resistor is used as a heat-generation element, the response is slow because the temperature increase of the medium is dependent on heat conduction. Furthermore, as an inherent problem, the refractive index change at higher frequencies cannot be attained.
Furthermore, a deflection element can include a thermal lens forming optical element including a photoreactive composition and an intensity distribution adjustment mechanism for irradiating the thermal lens forming optical element with a beam having a wedge-shaped light intensity distribution (refer to Japanese Patent Application Laid-Open No. 11-194373). The refractive index distribution can be formed in the thermal lens forming optical element with control light. Deflection of signal light can be realized by using the refractive index distribution. The signal light is different in wavelength from the control light. Although it is excellent to use the light to control the light, the above-described system causes a large loss of the control light for the intensity distribution adjustment mechanism that irradiates the thermal lens forming optical element with a beam having a wedge-shaped light intensity distribution. Furthermore, freely forming a wedge-shaped light intensity distribution is difficult. Thus, the optical path switching direction cannot be freely set.
Furthermore, a laser beam can be used to heat a material and change a refractive index of the material for deflection of the laser beam (refer to U.S. Pat. Nos. 4,776,677 and 4,585,301). The methods discussed in the above-mentioned prior art documents require a laser having a large beam size and a large power to cause desirable deflection of the laser beam. The method discussed in U.S. Pat. No. 4,776,677 can cause deflection of emitted light by heat generated by the emitted light itself. If the method discussed in U.S. Pat. No. 4,776,677 is used for optical deflection, the emitted light is almost absorbed to heat a material and change a refractive index. According to the principle, only a small-quantity of light can pass through the material.
The method discussed in U.S. Pat. No. 4,585,301 uses no electrical or mechanical mechanism. An optical switch disclosed in the U.S. Pat. No. 4,585,301 can change a refractive index with an emitted control beam and change an optical path of a signal beam. However, in this case, the laser is required to have a large power to change the refractive index because no lens is used for beam-condensation of the control beam and the signal beam. Furthermore, the apparatus has a larger size. Moreover, the deflection angle cannot be enlarged because there is no mechanism for expanding a refraction change region according to traveling of a beam as proposed in the present invention.
Both U.S. Pat. Nos. 4,776,677 and 4,585,301 disclose nothing about characteristic features of the present invention that includes a mechanism for separating and condensing non-deflection light and deflection light, and a mechanism for accurately separating non-deflection light and deflection light based on a difference in incidence angle between the non-deflection light and the deflection light entering into optical fibers used for an optical detection unit.